1 . Field of the Invention
This invention generally pertains to the field of fire alarms and detectors and more particularly concerns a method and apparatus for conveniently testing the operation of heat detector alarm installations.
2 . Background of the Invention
Early warning of fire in residential and commercial buildings has been proven to save numerous lives every year and has become a matter of national concern. For this purpose several different types of fire alarm systems are in use, designed to meet the requirements of various kinds of installations. Residential installations typically rely upon smoke detectors, which respond to the presence of air borne smoke particles generated in the early stages of combustion. However, smoke detectors can be unreliable in commercial and industrial environments due to the presence of other airborne materials, vapors and dusts produced in the normal course of commercial and industrial activity and which can falsely activate smoke detectors. Many commercial and industrial installations therefore depend upon heat detectors which are activated by certain changes in temperature indicative of a possible fire.
Most modern heat detectors incorporate either the rate of rise principle of operation or are of the rate compensated type. Each such type of detector is capable of sensing not simply the existence of an elevated temperature, but rather the rate of rise of the temperature of the air surrounding the detector so long as this rate exceeds preset limits. The temperature of air near a ceiling tends to rise rapidly in the event of a fire, and heat detectors incorporating the rate of rise or rate compensation feature are designed to respond to such rapid rise in temperature in order to discriminate against more gradual temperature increases unrelated to conflagrations. Rate compensated heat detectors, on the other hand, are a combination of fixed temperature and rate anticipation detector i.e., they activate an alarm simply upon reaching a given temperature during slow heat rise. During rapid heat rise, however, they are designed to account for the temperature lag between the detector temperature and air temperature. The temperature of the heat detector unit always lags behind the rising temperature of the surrounding air. This is because it takes a certain amount of time for heat transfer to occur from the ambient air to the heat sensor unit. The extent of this lag depends on how quickly the air temperature is rising, the lag being greater for a faster temperature rise of the air. Rate compensated heat detectors are constructed to compensate for this temperature lag, so as to trigger an alarm at a lower detector temperature if the temperature of the detector is rising rapidly, and trigger the alarm at a higher detector temperature if the rate of rise is slower.
Rate compensation detectors respond when the temperature of the air surrounding the device reaches a predetermined level, if the temperature rise is of a rate less than 5 degrees F/minute, and responds quickly thus eliminating temperature lag when the air temperature rise exceeds 5 degrees F/minute. A rate of rise detector, by contrast, responds when the detector temperature rises at a rate greater than 15 degrees F/minute but does not operate if the temperature rise is slower than 15 degrees F/minute. Some rate of rise heat detectors are combined with a fixed temperature detector. The fixed temperature portion of the combined rate of rise/fixed temperature heat detectors is sometimes activated by a fusible link made of a eutectic material, which can be a metallic alloy characterized by a low melting point. The eutectic alloy is selected to melt at the desired fixed temperature, and may be installed in such a way that an electrical circuit is closed when the fusible element melts. For example, a spring element can be held in a stretched condition so that upon melting of the eutectic element, the spring is released into contact with a second element to make an electrical connection. Eutectic alloy sensors are one shot devices, and must be replaced if once activated. Other models use a bi-metal arrangement which changes shape causing a contact closure at the desired temperature. Such detectors are self-restoring and so are reusable.
The various types of heat detectors are each available in several temperature ratings, designed to respond at different temperature ranges. The temperature classifications include the Low temperature range from 100 to 134 degrees Fahrenheit, the ordinary temperature range from 135 to 174 degrees Fahrenheit, the Intermediate range from 175 to 249 degrees, and several still higher temperature ranges. The great majority of heat detectors currently in use, however, fall within the Ordinary temperature range, i.e. they activate at about 135 degrees Fahrenheit.
Each heat detector has a radius of effective coverage. This radius varies from one heat detector model to another, and typically is between 25 feet and 50 feet. A typical installation requires a number of heat detectors installed in a grid pattern on the ceiling of the structure to be protected. The spacing between the detectors is determined by the effective coverage capability of each unit. A large commercial or industrial space, such as a warehouse, may have a considerable number of heat detectors. Furthermore, such spaces commonly have high ceilings, which places the heat detectors out of easy reach.
At present, only makeshift methods exist for the operational testing of heat detectors, if such testing is done at all. Commonly employed heat sources include the use of hair dryers, heat guns and heat lamps. A ladder must be placed under each heat detector and the heat source is hand carried up the ladder to test the detector. Long extension cords are typically required by this approach. Clearly, this is a cumbersome, time consuming and ineffective approach to the testing of heat detectors, with the result that too often heat detectors go untested over extended periods, in spite of annual testing requirements by industrial and commercial codes.
A need exists for an efficient and reliable method for testing heat detector installations.